JRuby classes generated by "jrubyc" are different that generated by "jrubyc --javac".
If you want to implement Java inteface in JRuby use "jrubyc --javac" command instead of "jrubyc".

[1 case]
jrubyc --javac:
public class SimpleImplementation extends RubyObject implements SimpleJavaInterface { (...) }

[2 case]
jrubyc:
public class SimpleImplementation extends AbstractScript { (...) }

[2 case] generete error:
"incompatible types" for code:

SimpleJavaInterface sji = new SimpleImplementation();



=====================================================
 how to run code:
=====================================================
	update variables in run.sh
	bash run.sh



...
output:

=====================================================
 jruby jar: [/home/rakk/.m2/repository/org/jruby/jruby/1.6.0/jruby-1.6.0.jar]
=====================================================
=====================================================
 jrubyc --javac
=====================================================
...............compile SimpleImplementation.rb
Generating Java class SimpleImplementation to output_jrubyc_with_javac/SimpleImplementation.java
javac  -d output_jrubyc_with_javac -cp /home/rakk/dev/bin/langs/jruby/lib/jruby.jar:SimpleJavaInterface-1.0.jar:/home/rakk/.m2/repository/org/jruby/jruby/1.6.0/jruby-1.6.0.jar:. output_jrubyc_with_javac/SimpleImplementation.java
...............we get...
output_jrubyc_with_javac/SimpleImplementation.java
(...)

public class SimpleImplementation extends RubyObject implements SimpleJavaInterface {
    private static final Ruby __ruby__ = Ruby.getGlobalRuntime();
    private static final RubyClass __metaclass__;

(...)
output_jrubyc_with_javac/SimpleImplementation.class
(...)
public class SimpleImplementation extends RubyObject
    implements SimpleJavaInterface
{

    private SimpleImplementation(Ruby ruby, RubyClass rubyclass)
(...)
=====================================================
 jrubyc --java
=====================================================
...............compile SimpleImplementation.rb
Generating Java class SimpleImplementation to output_jrubyc_with_java/SimpleImplementation.java
...............we get...
output_jrubyc_with_java/SimpleImplementation.java:
(...)

public class SimpleImplementation extends RubyObject implements SimpleJavaInterface {
    private static final Ruby __ruby__ = Ruby.getGlobalRuntime();
    private static final RubyClass __metaclass__;

(...)
=====================================================
 jrubyc
=====================================================
...............compile SimpleImplementation.rb
Compiling SimpleImplementation.rb to class SimpleImplementation
...............we get...
output_jrubyc/SimpleImplementation.class:
(...)
import org.jruby.runtime.*;
import org.jruby.runtime.builtin.IRubyObject;

public class SimpleImplementation extends AbstractScript
{

    private static void setPosition(ThreadContext threadcontext, int i)
    {
        threadcontext.setFileAndLine("SimpleImplementation.rb", i);
(...)